Memory that retains its data even when operation power is not available is classified as nonvolatile memory. Examples of non-volatile memory are nvSRAM, F-RAM, electrically erasable programmable read-only memory (EEPROM), and flash memories. This class of memory may be used in applications in which critical data must be stored after power is removed, or when power is interrupted during operation. An example of power interruptions is the hot plugging of cards in servers, industrial computers, and medical equipment. Several submodules can be unplugged while the equipment is operating (that is, their power can be interrupted) and new submodules can be plugged in (hot plugged because power is present in the equipment) without losing any critical data and/or operations to the equipment.
Conventional static random access memory (SRAM) loses its content when powered down, and is classified as volatile memory. The memory is volatile because there is no data when power is restored to the device. Another example of volatile memory is dynamic random access memory (DRAM) which is sometimes used in desktop computers and laptops.
The non-volatile SRAM (nvSRAM) is a class of NV memory that combines SRAM interface and speed with the non-volatility of a NV memory cell. The nvSRAM may achieve high speed and non-volatile storage at low cost compared to alternative solutions, such as large supercapacitors and batteries to retain data on devices when power is interrupted. These applications include but are not limited to smart meters, servers, industrial programmable logic controllers (PLCs), gaming, multifunction printers, and storage units.
In general, if there is a possibility of power to the nvSRAM being lost, the bit of data can be transferred from the SRAM portion of the cell to the NV portion for storage. At a later time, the bit of data can be returned from the NV portion back to the SRAM portion in a recall operation. SRAM may then transmit the bit of data to the exterior environment, if desired.